Solar energy conversion to electricity and fuels is the prime candidate for supplanting fossil fuels, which cause pollution that harms human health and contribute to global climate change by emitting carbon dioxide. To achieve this aim, different technologies suitable for various purposes have been emerging in the past few decades. In most of these technologies, a single semiconductor absorbs sunlight and then transports the photo-generated carriers to the external circuit or to the junction with the electrolyte. However, some light absorbers, such as dye molecules in dye-sensitized solar cells, are poor charge conductors. Other light absorbers, such as typical low-gap semiconductors, are not stable enough to be exposed to electrolytes, and therefore are not suitable for use in photocatalytic technologies.
One strategy to overcome these barriers is to separate light absorption and charge conduction components of the device. For this purpose, the light absorber could inject the photogenerated carrier into a wide-gap semiconductor with higher conductivity (commonly dubbed a transparent conducting oxide (TCO)) that does not absorb visible light, and therefore does not interfere with light-absorption and carrier generation. Such technologies require the use of suitable TCOs with high enough conductivities.
Common n-type electron conductors are TiO2 and ZnO, which are widely used in many technologies such as n-type dye-sensitized solar cells. However, finding an effective p-type TCO is still an active area of research, as available p-type TCOs generally have lower conductivities than their n-type counterparts. A widely used p-type TCO is nickel oxide, which has a band gap of 3.6-4.6 eV and is intrinsically p-type due to Ni vacancies in its lattice structure. Producing highly conducting NiO samples—and preferably crystals that maintain the rock-salt structure of NiO with minimal structural distortion—for use as p-type TCOs has attracted extensive interest in the past decade.
Thus, highly conductive p-type TCOs that have a rock-salt structure are therefore highly desirable.